1. Field of the Invention
This invention relates to the operation of a pressure vessel and apparatus for carrying out such operation.
2. Description of the Prior Art
Although, for sake of clarity and brevity, this invention will be described in respect of the solution polymerization of ethylene, it is to be understood that this invention applies generally to curvilinear pressure vessels that operate at an elevated pressure, e.g., at least about 1,000 psig, and that contain a bed of particulate material through which a process fluid is to flow in a substantially uniform manner. For example, this invention can be applied to adsorbent beds, catalyst beds, and fixed beds such as those used in processes such as polymer formation.
Heretofore, linear high density polyethylene (HDPE) has been formed by polymerizing ethylene while dissolved in a solvent such as hexane. The resulting solvent solution also contains a polymerization catalyst such as the combination of TiCl4 and VOCl3. The polymerization reaction is carried out in a single liquid phase containing at least the above components using a series of stirred reactors followed by a tubular (plug flow) reactor. Downstream of the last reactor a catalyst deactivator such as acetylacetone is injected into the solution, and the resulting mixture introduced into an adsorption vessel which is a pressure vessel. In the adsorber catalyst compounds and decomposition components of the deactivator are adsorbed from the single phase solution. The polymerization reaction is carried out at an elevated temperature of from about 150 to about 280 degrees Centigrade (C.) at a pressure of from about 2,000 to about 4,000 psig. Thus, the adsorption step of this process is carried out at a very high pressure, and this requires, for sake of capital costs, an adsorber configuration that is curvilinear, typically spherical.
The adsorbent material used in this pressure vessel is typically a particulate material. These particles adsorb from the single phase liquid solution various catalyst moieties such as titanium compounds, vanadium compounds, and by-products of the decomposition of the catalyst deactivator. The adsorbent for the exemplary HDPE process above is typically activated alumina particles such as alumina spheres about 1.7 millimeters in diameter. As these particles adsorb catalyst and deactivator compounds from the single phase liquid passing through the adsorbent bed, they change in color, typically from an initially white color to varying shades of gray, to black, the darker the adsorbent particle, the greater the extent of adsorption of the aforementioned materials by that particle.
The particulate adsorbent, when initially loaded into the adsorber, is gravity poured through a nozzle opening in an upper portion of the vessel down into the interior of the vessel, and allowed to pile up therein to a predetermined level. This invariably leaves an adsorbent bed in the vessel with an uneven upper surface, typically an inverted conical surface that rises to a peak approaching, but below, the opening through which it was poured. This conical pile of particulates normally piles up at its natural angle of repose, e.g., about a 30 degree angle from the horizontal for the alumina particles used in an HDPE adsorber.
After the conical pile of adsorbant is formed in the vessel, the vessel is put into operation and the high temperature, high pressure, single phase solution aforesaid is passed into the nozzle in the vessel for contact with the adsorbent bed. This nozzle is typically an upstanding conduit whose long axis is substantially vertical. The single phase liquid solution is then passed into the nozzle at an angle that is transverse, e.g., a 90 degree angle, to the long axis of the conduit so that the solution must make a sharp turn downward in order to enter the interior of the vessel where the adsorbent bed lies.
In the exemplary HDPE process, as with many other processes, a conventional plug flow reactor is employed upstream of the adsorber to accomplish product uniformity with a uniform residence time distribution for the reactants in that reactor. By “plug flow,” what is meant is substantially uniform fluid velocity distribution across a transverse cross-section of a reactor, and maintenance of that flow as that fluid passes longitudinally through the reactor from its entrance to its exit. This gives all portions of that process fluid essentially uniform residence time in the reactor. This same plug flow concept can be applied to other vessels, including, but not limited to, adsorbent vessels.
The curvilinear shape of a high pressure adsorber, the conical shape of the adsorbent bed in the adsorber, and the right angle turn the single phase solution must make after it enters the nozzle of the adsorber, all work against achieving anything like plug flow of the solution through the adsorbent bed. This causes mal-distribution of solution as it passes to and through the bed, which results in channeling of solution through localized portions of the bed. This channeling causes underutilization of the adsorbent throughout substantial volumes of that bed, while other portions, where the channeling occurs, are forced to treat too much solution. The result of channeling can be seen in a used alumina bed height profile wherein some portions (groups) of alumina particles are black, while other groups are still white, indicating no adsorption at all.
The HDPE process must be carried out in a single phase solution. If two phases (a polymer rich phase and a solution rich phase) were allowed to form, a phenomenon known in the art as “frosting” or “two-phasing” occurs wherein solid polymer forms in the interior of the reactors and adsorbers, and deposits there. Process conditions such as temperature, pressure, and mass composition of the single phase solution stream can determine whether the stream will stay in the single phase or move toward two-phasing. If two-phasing is allowed to continue unchecked, the vessels in which it is occurring will eventually plug up with solid polyethylene thereby requiring shut down of the plant, and clean up of at least the affected vessels, a costly event in terms of lost production and clean-up costs.
Mal-distribution of single phase solution flow through an adsorber bed can cause two-phasing and polymer deposition in the bed due to an undesired change in pressure where the solution channels through the bed. This can lead to plugging of at least sections of the bed, up to, and including, the entire bed if left unchecked. This then necessitates a premature and costly shut down of the adsorber and replacement of the bed with fresh adsorbent.
Thus, it is highly desirable to operate an HDPE adsorber in a manner that more closely approaches plug flow through the particulate bed. This invention does just that by attacking both the distribution of the process fluid over the bed, and the configuration of the uneven, upper surface of the bed itself. This premise applies as well to other bed containing pressure vessels such as catalyst containing vessels, and the like.